This invention pertains to apparatus and methods for inspecting a sample, such as a semiconductor wafer or photomask, using an optical inspection system. It also pertains to apparatus and methods for enhancing the detection of defects in such optical inspection systems.
Generally, the industry of semiconductor manufacturing involves highly complex techniques for fabricating integrating circuits using semiconductor materials. Due to the large scale of circuit integration and the decreasing size of semiconductor devices, decreasingly small defects, such as a particle, can cause the devices to fail. Defect inspection procedures are therefore critical to maintain quality control. Since the inspection procedures are an integral and significant part of the manufacturing process, the semiconductor industry is constantly seeking more accurate and efficient testing procedures.
Various inspection systems are used within the semiconductor industry to detect defects on a semiconductor device or wafer. One type of inspection tool is an optical inspection system. In optical inspection systems, one or more radiation beams are directed towards the semiconductor wafer and a reflected and/or scattered beam is then detected. The detected beam is used to then generate a detected electrical signal or an image, and such signal or image is then analyzed to determine whether defects are present on the wafer.
In certain applications, it is also frequently required that the inspection system have configurable illumination and imaging designs. The illumination and imaging configuration will be set to optimize the capture of different characteristics of defects or defect types. That is, different illumination and imaging configurations are more suitable for different types of defect inspections. Two broad categories of inspection configurations include bright field and dark field inspection. In general, the illumination and collection beam profiles are adjusted to achieve different inspection modes. In other words, different portions or angles of the incident or collection beam are blocked or transmitted.
For a dark field inspection, a portion of the illumination beam profile is typically blocked so that only a portion of the available illumination, for example a ring of illumination, is passed through to the wafer. A corresponding portion of the collection beam profile is then blocked so that only scattered light is collected. That is, blocked portions of the illumination beam correspond to unblocked portions of the collection beam, while unblocked portions of the illumination beam correspond to blocked portions of the collection beam. Both the illumination and the collection adjustments implement binary masks. That is, portions of the beam are totally blocked by the mask, while other portions of the beam pass unimpeded through the mask. Although these mask or aperture techniques have proved to be effective in many applications, they have limited sensitivity enhancement with respect to low contrast structures on the wafer, such as high aspect ratio vias.
In light of the foregoing, improved mechanisms for inspecting a sample are needed to achieve higher sensitivity for low contrast structures and defects and, accordingly to facilitate defect detection.